Chapter 3 Transport Layer slides are modified from J. Kurose & K. Ross CPE 400 / 600 Computer Communication Networks Lecture 12

Transport Layer 2 Chapter 3 outline r 3.1 Transport-layer services r 3.2 Multiplexing and demultiplexing r 3.3 Connectionless transport: UDP r 3.4 Principles of reliable data transfer r 3.5 Connection-oriented transport: TCP r 3.6 Principles of congestion control r 3.7 TCP congestion control

Transport Layer 3 Principles of Congestion Control Congestion: r informally: “too many sources sending too much data too fast for network to handle” r different from flow control! r manifestations:  lost packets (buffer overflow at routers)  long delays (queueing in router buffers) r a top-10 problem!

Transport Layer 4 Causes/costs of congestion: scenario 1 r two senders, two receivers r one router, infinite buffers r no retransmission r large delays when congested r maximum achievable throughput unlimited shared output link buffers Host A in : original data Host B out

Transport Layer 5 Causes/costs of congestion: scenario 2 r one router, finite buffers r sender retransmission of lost packet finite shared output link buffers Host A in : original data Host B out ' in : original data, plus retransmitted data

Transport Layer 6 Causes/costs of congestion: scenario 2 r always: (goodput) r “perfect” retransmission only when loss: r retransmission of delayed (not lost) packet makes larger (than perfect case) for same in out = in out > in out “costs” of congestion: r more work (retrans) for given “goodput” r unneeded retransmissions: link carries multiple copies of pkt R/2 in out b. R/2 in out a. R/2 in out c. R/4 R/3

Transport Layer 7 Causes/costs of congestion: scenario 3 r four senders r multihop paths r timeout/retransmit in Q: what happens as and increase ? in finite shared output link buffers Host A in : original data Host B out ' in : original data, plus retransmitted data

Transport Layer 8 Causes/costs of congestion: scenario 3 Another “cost” of congestion: r when packet dropped, any “upstream transmission capacity used for that packet was wasted! HostAHostA HostBHostB o u t

Transport Layer 9 Approaches towards congestion control End-end congestion control: r no explicit feedback from network r congestion inferred from end-system observed loss, delay r approach taken by TCP Network-assisted congestion control: r routers provide feedback to end systems  single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM)  explicit rate sender should send at Two broad approaches towards congestion control:

Transport Layer 10 Lecture 12 outline r 3.6 Principles of congestion control r 3.7 TCP congestion control

Transport Layer 11 TCP congestion control: additive increase, multiplicative decrease r Approach: increase transmission rate (window size), probing for usable bandwidth, until loss occurs m additive increase: increase CongWin by 1 MSS every RTT until loss detected m multiplicative decrease: cut CongWin in half after loss congestion window size time Saw tooth behavior: probing for bandwidth

Transport Layer 12 TCP Congestion Control: details r sender limits transmission: LastByteSent-LastByteAcked  CongWin r Roughly,  CongWin is dynamic, function of perceived network congestion How does sender perceive congestion?  loss event = timeout or 3 duplicate acks  TCP sender reduces rate ( CongWin ) after loss event three mechanisms:  AIMD  slow start  conservative after timeout events rate = CongWin RTT Bytes/sec

Transport Layer 13 TCP Slow Start  When connection begins, CongWin = 1 MSS  Example: MSS = 500 bytes & RTT = 200 msec  initial rate = 20 kbps r available bandwidth may be >> MSS/RTT  desirable to quickly ramp up to respectable rate r When connection begins, increase rate exponentially fast until first loss event  double CongWin every RTT  done by incrementing CongWin for every ACK received

Transport Layer 14 TCP Slow Start (more) r Summary: initial rate is slow but ramps up exponentially fast Host A one segment RTT Host B time two segments four segments

Transport Layer 15 Refinement: inferring loss r After 3 dup ACKs:  CongWin is cut in half  window then grows linearly r But after timeout event:  CongWin instead set to 1 MSS;  window then grows exponentially  to a threshold, then grows linearly  3 dup ACKs indicates network capable of delivering some segments  timeout indicates a “more alarming” congestion scenario Philosophy:

Transport Layer 16 Refinement Q: When should the exponential increase switch to linear? A: When CongWin gets to 1/2 of its value before timeout. Implementation: r Variable Threshold r At loss event, Threshold is set to 1/2 of CongWin just before loss event

Transport Layer 17 Summary: TCP Congestion Control  When CongWin is below Threshold, sender in slow- start phase, window grows exponentially.  When CongWin is above Threshold, sender is in congestion-avoidance phase, window grows linearly.  When a triple duplicate ACK occurs, Threshold set to CongWin/2 and CongWin set to Threshold.  When timeout occurs, Threshold set to CongWin/2 and CongWin is set to 1 MSS.

Transport Layer 18 TCP sender congestion control StateEventTCP Sender ActionCommentary Slow Start (SS) ACK receipt for previously unacked data CongWin = CongWin + MSS, If (CongWin > Threshold) set state to “Congestion Avoidance” Resulting in a doubling of CongWin every RTT Congestion Avoidance (CA) ACK receipt for previously unacked data CongWin = CongWin+MSS * (MSS/CongWin) Additive increase, resulting in increase of CongWin by 1 MSS every RTT SS or CALoss event detected by triple duplicate ACK Threshold = CongWin/2, CongWin = Threshold, Set state to “Congestion Avoidance” Fast recovery, implementing multiplicative decrease. CongWin will not drop below 1 MSS. SS or CATimeoutThreshold = CongWin/2, CongWin = 1 MSS, Set state to “Slow Start” Enter slow start SS or CADuplicate ACK Increment duplicate ACK count for segment being acked CongWin and Threshold not changed

Transport Layer 19 TCP throughput r What’s the average throughout of TCP as a function of window size and RTT?  Ignore slow start r Let W be the window size when loss occurs. r When window is W, throughput is W/RTT r Just after loss, window drops to W/2, throughput to W/2RTT. r Average throughout:.75 W/RTT

Transport Layer 20 TCP Futures: TCP over “long, fat pipes” r Example: 1500 byte segments, 100ms RTT, want 10 Gbps throughput r Requires window size W = 83,333 in-flight segments r Throughput in terms of loss rate:  ➜ L = 2· Wow r New versions of TCP for high-speed

Transport Layer 21 Fairness goal: if K TCP sessions share same bottleneck link of bandwidth R, each should have average rate of R/K TCP connection 1 bottleneck router capacity R TCP connection 2 TCP Fairness

Transport Layer 22 Why is TCP fair? Two competing sessions: r Additive increase gives slope of 1, as throughout increases r multiplicative decrease decreases throughput proportionally R R equal bandwidth share Connection 1 throughput Connection 2 throughput congestion avoidance: additive increase loss: decrease window by factor of 2 congestion avoidance: additive increase loss: decrease window by factor of 2

Transport Layer 23 Fairness (more) Fairness and UDP r Multimedia apps often do not use TCP  do not want rate throttled by congestion control r Instead use UDP:  pump audio/video at constant rate, tolerate packet loss r Research area: TCP friendly Fairness and parallel TCP connections r nothing prevents app from opening parallel connections between 2 hosts. r Web browsers do this r Example: link of rate R supporting 9 connections;  new app asks for 1 TCP, gets rate R/10  new app asks for 11 TCPs, gets R/2 !

Transport Layer 24 Chapter 3: Summary r principles behind transport layer services:  multiplexing, demultiplexing  reliable data transfer  flow control  congestion control r instantiation and implementation in the Internet  UDP  TCP Next: r leaving the network “edge” (application, transport layers) r into the network “core”